From: fche@redhat.com (Frank Ch. Eigler)
To: Arkady <arkady.miasnikov@gmail.com>
Cc: David Smith <dsmith@redhat.com>, systemtap <systemtap@sourceware.org>
Subject: Re: Unsafe mode for probes
Date: Thu, 09 Aug 2018 20:07:00 -0000 [thread overview]
Message-ID: <87h8k3tibs.fsf@redhat.com> (raw)
In-Reply-To: <CANA-60owZN-ksNwHRNc4BwesinQBqyxn2R20g8OuHwgBP7hADQ@mail.gmail.com> (Arkady's message of "Tue, 7 Aug 2018 12:19:59 +0300")
arkady.miasnikov wrote:
> My typical probe looks like this
>
> probe syscall.accept4.return ?
> {
> %{HIT_MAP_INC(HIT_MAP_SYSCALL_ACCEPT4_RETURN)%}
> send_incident_result(%{INCDENT_TYPE_SYSCALL_ACCEPT4%}, $return);
> }
> [...]
> The actual assembler of the probe will contain quite a bit of code
> handling local variables, call to lock, etc
> For short probes I can shave about 20% of CPU cycles.
A blanket "unsafe" probe cannot make do with no checks at all, e.g.
a proper context structure allocation for temporary values, etc.
It's a matter of detail - which checks particularly should one skip?
It is not unlikely that some of that initialization / lock business in
the probe prologue could be elided entirely with some more cleverness
during translation. Like a new pragma for embedded-C functions that
skip the context-struct based api, which in turn could make it
unnecessary to have a context struct at all for that probe. Stuff
like that - but it takes analysis to figure out which is needed
and which is not. stap -p3 ftw.
- FChE
next prev parent reply other threads:[~2018-08-09 20:07 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-05 10:14 Arkady
2018-08-06 17:37 ` David Smith
2018-08-07 9:20 ` Arkady
2018-08-09 20:07 ` Frank Ch. Eigler [this message]
2018-08-11 4:06 ` Arkady
2018-08-11 15:37 ` Frank Ch. Eigler
2018-08-12 15:16 ` Arkady
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